Biodiesel are long-chain fatty acid ester substances generated by transesterification reaction of biological oil and fat feedstock. It is a novel renewable energy resource without pollution, and has broad application prospects in oil and fat industry. The combustion properties of biodiesel are comparable to traditional petroleum diesel, and the harmful substances in the exhaust emitted from an engine in which biodiesel is combusted is 50% reduced when compared to traditional petroleum diesel. Currently, extensive attention has been drawn to the investigations and applications of biodiesel.
Currently, biodiesel is primarily produced by chemical processes, i.e. using animal/plant oil and fat together with some low carbon alcohols (methanol or ethanol) to conduct, transesterification reaction under the catalysis of acidic or alkaline catalysts, so as to generate corresponding fatty acid methyl ester or ethyl ester. There exist the following disadvantages when using chemical processes to prepare biodiesel: (1) the free fatty acids and water in the oil and fat feedstock severely affect the progression of the reaction; (2) the poor solubility of methanol in oil and fat would easily result in the formation of emulsion, such that the subsequent processing procedure will become complicated; and (3) the amount of methanol used according to the requirements of the process greatly exceeds the molar ratio of the reaction, and the recovery of the excess methanol increases the energy consumption during the process.
A process using a biological enzyme for synthesizing biodiesel has the following advantages: mild reaction conditions, no pollutant emitted, and broad applicability for various oil and fat feedstock. Such process is in accordance with the green chemistry trend, and thus is getting increasing attention. However, in conventional lipase-catalyzing processes of converting oil and fat feedstock for the preparation of biodiesel, there exists the following problem, when the water content in the oil and fat feedstock is greater than 0.5% (based on the weight of the oil), the acid value of the biodiesel product after the reaction is generally higher than 0.5 mg KOH per gram of oil. This does not fulfill the acid value standard required for biodiesel quality control, and thus subsequent processing procedures involving complicated alkaline neutralization are required. However, such subsequent, processing procedures using alkaline neutralization to decrease the acid value affects the yield of the product, and can bring about pollution problems. Accordingly, it is desired to develop an efficient and environment-friendly method for preparing biodiesel.